1. Field of the Invention
The present invention relates to a throttle device for an internal combustion engine.
2. Description of Related Art
Throttle devices having a butterfly-type throttle valve are known as disclosed in Laid-Open Japanese Utility Model Publications Nos. 48-41916, 53-142617, and 1-85433. Those devices are intended to adjust the flow rate of intake air flowing in the intake air passage by altering the shape of the throttle valve according to a degree of opening of the butterfly-type throttle valve. The flow rate of the intake air flowing into the throttle device is measured by an air flow meter.
In recent years, the size of air intake systems has been reduced by mounting an air flow meter proximate to the throttle valve. However, upstream from and proximate to the throttle valve, the flow velocity of the intake air flowing at the upstream half side of the throttle valve is higher than that of the intake air flowing at the downstream half side thereof. That is, the flow velocity of the intake air is different according to the position in a section of the intake air passage. Therefore, it is difficult for an air flow meter positioned proximate to and upstream from the throttle valve to measure the flow rate of the intake air with high accuracy. Further, when the intake air collides with the throttle valve, the air flow becomes turbulent in the periphery of the upstream half of the throttle valve, thus generating an eddy flow. Thus, it is difficult to measure the flow rate of the intake air with high accuracy.
The above throttle devices are intended not to measure the flow rate of the intake air with high accuracy but to adjust the flow rate of the intake air by altering the shape of the throttle valve.
It is possible to restrict a throttle valve-caused fluctuation in the flow velocity of the intake air and the generation of a turbulent flow, by installing the air flow meter at a position upstream and distant from the throttle valve. However, such a construction causes the throttle device to be long and large.